1. Field of the Invention
Embodiments of the present invention are directed to an optical sensor capable of measuring two analytes simultaneously with a single indicator system. In preferred embodiments, the sensor comprises a fluorescent dye having acid and base forms that facilitate ratiometric pH sensing, wherein the dye is further associated with a glucose binding moiety and configured to generate a signal that varies in intensity with the concentration of glucose.
2. Description of the Related Art
There has been an on-going effort over many years to use fluorescence techniques to measure polyhydroxyl compound (e.g., glucose) concentration in bodily fluids. But despite the effort, no practical system has been developed and commercialized for in vivo monitoring. Several attempts have been made to detect glucose by fluorescence using dyes associated with boronic acid groups. Boronate moieties bind glucose reversibly. When boronic acid functionalized fluorescent dyes bind glucose, the properties of the dye are affected, such that a signal related to the concentration of glucose may be generated and detected. These changes have been used in the past to measure glucose concentration.
Russell (U.S. Pat. Nos. 5,137,833 and 5,512,246) used a boronic acid functionalized dye that bound glucose and generated a signal related to the glucose concentration. James et al. (U.S. Pat. No. 5,503,770) employed a similar principle, but combined a fluorescent dye, an amine quenching functionality, and boronic acid in a single complex. The fluorescence emission from the complex varied with the amount of glucose binding. Van Antwerp et al. (U.S. Pat. Nos. 6,002,954 and 6,011,984) combined features of the previously cited references and also disclosed a device purported to be implantable. A. E. Colvin, Jr. (U.S. Pat. No. 6,304,766) also disclosed optical-based sensing devices for in situ sensing in humans that utilize boronate-functionalized dyes.
Certain measurable parameters using blood or bodily fluid, such as pH and concentrations of O2, CO2, Na+, K+, and polyhydroxyl compounds, like glucose, have been determined in vivo. The ability to do these measurements in vivo is important because it is necessary to make frequent determinations of such analytes when monitoring a patient. Typically, one sensor for each analyte has been placed in a patient's blood vessel(s). If it is desired to measure several analytes, a plurality of sensors is often required, which can cause attendant discomfort to the patient and complexity of the electronic monitoring equipment.
In an effort to solve the design problems posed by the limitation in physical dimension for in vivo monitoring, others have incorporated different dyes into one device to get simultaneous readings of two parameters. For example, Alder et al. (U.S. Pat. No. 5,922,612) disclosed a method for optical determination of pH and ionic strength of an aqueous sample using two different dyes on one sensor. Gray et al. (U.S. Pat. No. 5,176,882) taught the use of a fiber optic device incorporating a hydrophilic polymer with immobilized pH sensitive dye and potassium or calcium sensitive fluorescent dyes to measure the analyte concentration in conjunction with pH. In U.S. Pat. No. 4,785,814, Kane also disclosed the use of two dyes embedded in a composite membrane for the simultaneous measurements of pH and oxygen content in blood. However, incorporation of multiple dyes into a single sensor complicates the manufacture of such sensors.
Besides the foregoing problems associated with separate indwelling sensors for each analyte being monitored, particularly in the intensive care setting, and multiple dye sensors, another problem associated with many dye-based analyte sensors is pH sensitivity. A slight change in pH may modify or attenuate fluorescence emissions, and cause inaccurate readings. This problem is particularly acute for monitoring blood glucose levels in diabetic patients, whose blood pH may fluctuate rapidly. Since accurate blood glucose level measurements are essential for treating these patients, there is a significant need for a glucose sensor that facilitates real-time correction of the pH effect without requiring separate indwelling pH and analyte sensors, or sensors having multiple dyes.
Ratiometric pH determination using fluorescent dye(s) is known. Given a fluorophore that has an acid and base form, the ratio of the emission intensity of the two forms can be used as a measure of the pH that is insensitive to fluorophore concentration. See e.g., U.S. Patent Publication No. 2005/0090014 which describes an HPTS-derived pH sensitive dye (incorporated herein in its entirety by reference); Niu C. G. et al. 2005 Anal. Bioanal. Chem. 383(2):349-357, which describes a pH-sensitive dye meso-5,10,15,20-tetra-(4-allyloxyphenyl)porphyrin (TAPP) as an indicator, and a pH-insensitive benzothioxanthene derivative as a reference, for fluorescence ratiometric measurement; Turner N. G. et al. 1998 J. Investig. Dermatol. Symp. Proc. August 3(2):110-3, which discloses dual-emission ratiometric imaging using the fluorophore, carboxy seminaphthorhodafluor-1, which displays a pH-dependent shift in its emission spectrum; and Badugu R. et al. 2005 Talanta 66:569-574, which describes the use of 6-aminoquinolinium boronic acid dyes that show spectral shifts and intensity changes with pH in a wavelength-ratiometric manner.
However, despite the inventor's recognition of a substantial unmet need for a sensor adapted to provide continuous intravascular monitoring of pH and glucose, wherein the glucose measurement may be corrected for pH effects, no one has disclosed or even suggested using a sensor comprising a single fluorophore that exhibits properties suitable to make a ratiometric pH measurement that is independent of the fluorophore concentration, where the same fluorophore is functionalized to bind glucose and generate a signal the intensity of which is related to the glucose concentration.